Repeated exposure to specific phobia stimuli, Trimethylthiazoline, promoted fear extinction, hippocampal neurogenesis, and microglia activation in rats

Specific phobia (SP) is an anxiety disorder characterized by an overwhelming and irrational fear of specific objects, situations, or activities. In clinical practice, exposure therapy is considered one of the most effective treatments for SP. This approach involves repeated exposure to fear-inducing stimuli to facilitate fear extinction. While fear extinction and systematic desensitization are learning-dependent processes, the role of hippocampal neurogenesis remains unexplored in this context. In this study, we employed an animal model of repeated exposure to Trimethylthiazoline (TMT), a predator odor derived from fox feces, to investigate the potential involvement of hippocampal neurogenesis in exposure therapy. Twenty adult male Sprague Dawley rats were randomly assigned to either a control group (exposed to distilled water) or a TMT-exposed group (exposed to TMT) repeatedly within 14 days. Serum samples were collected 2–3 days following treatment. Neurogenesis and cell proliferation in brain tissues were analyzed via immunohistochemistry. The results demonstrated that repeated TMT exposure facilitated fear extinction without inducing anxiety- or depression-like behaviors. Additionally, TMT exposure enhanced neurogenesis by promoting the differentiation of neuronal progenitors into immature neurons. Microglial activation, implicated in various stages of adult neurogenesis, also increased following TMT exposure. These findings provide preliminary evidence supporting the therapeutic mechanism, showing that repeated TMT exposure not only facilitates fear extinction but also promotes neurogenesis and microglial activation. This study suggests the potential roles of neurogenesis and microglial activation in fear extinction or systematic desensitization, and identifies novel targets for future therapeutic development in SP.